Whispering-gallery mode (WGM) microcavities with the merits of small mode volumes and high quality (Q) factors have attracted great research interests as potential low-power-consumption light sources for photonic integration. We propose and demonstrate deformed square microcavity lasers with the flat sidewalls replaced by circular arcs as converge mirrors to control the WGMs inside the laser cavity. The ray dynamic analysis results indicate that the circular-sides can confine the light rays with stable islands, although full chaotic dynamics are observed under certain deformation. With the numerical simulation of the circular-side square microcavities, ultrahigh-Q modes are obtained owing to the elimination of the scattering losses from the vertices, and a reduction of mode Q factors due to the chaotic ray dynamics is also observed. Different transverse modes have distinct light trajectories, which results in a difference of the effective roundtrip length and a controllable transverse mode interval. Low threshold lasing is achieved experimentally due to the high Q factors of the WGMs. The lasing spectra can be engineered by designing the cavity geometry for the waveguidecoupled circular-side square microcavity lasers. The robust structure and ultrahigh-Q of the waveguide-coupled microlasers provide a potential solution for the compact light sources in photonic integrated circuits.

Lasing mode control and direct modulation characteristics have been investigated for waveguide-coupled unidirectional-emission square microcavity lasers. A quasi-analytical model is introduced to analyze the mode field distributions and quality (<i>Q</i>) factors for the confined modes inside the square optical microcavities with directly coupled waveguide, where high-<i>Q</i> whispering-gallery-like (WG-like) modes are induced by the mode coupling between doubly-degenerate modes. AlGaInAs/InP waveguide-coupled unidirectional-emission square microcavity lasers are fabricated by using standard planar technology, and electrically-injected lasing is realized at room temperature. The lasing modes are controlled by properly designing the lasing cavity, output waveguide and injection pattern. Dual-transverse-mode lasing with a tunable wavelength interval from 0.25 to 0.39 nm is realized by using a spatially selective current injection to modulate the refractive index, as the mode field distributions of different transverse are spatially separated. The wavelength interval can be further increased to a few nanometers by reducing the cavity size and replacing the flat sidewalls with circular arcs. The field distributions of WG-like modes distribute uniformly in square microcavity, which avoid the burning-induced carrier diffusion in high-speed direct modulation. A small-signal modulation 3dB bandwidth exceeding 16 GHz, and an open eye diagram at 25 Gb/s are demonstrated for the high-speed direct modulated square microcavity laser.

The characteristics of integrated lasers consisting of a Fabry-Pérot (FP) cavity with one side connected to a whisperinggallery mode (WGM) microcavity are reported, in which the WGM microcavity acts as a resonant reflector for the FP cavity with its reflectivity capable of being modulated. The mode coupling between the WGM and FP mode would clamp the lasing mode around the wavelength of the WGM and suppress additional side modes. Single mode lasing with the side mode suppression ratio higher than 40 dB is realized for an integrated laser with the FP cavity directly connecting to one vertex of a square microcavity. The wavelength tunability is further demonstrated by varying the bias currents into the square cavity and the FP cavity regions. In addition to the lasing characteristics of single mode operation, we also report the high speed modulation characteristics of the integrated laser.

Due to the merits of small size and low power consumption, microdisk lasers have been widely investigated as a potential light source for photonic integrated circuits. We investigate the dynamic and mode characteristics for a directional-emission microdisk laser subject to optical injection. At the free-running state, single-mode operation at 1540.16 nm with side-mode suppression ratio of 35 dB is realized for an 8-μm-radius microdisk laser connected with a 2-μm-width output waveguide at 7 mA. Under the injection locking state with the injection optical power of 2 mW, the 3-dB bandwidth of the small signal modulation response is enhanced from 3.4 to 13.7 GHz for the microdisk laser biased at 7 mA. Furthermore, the nonlinear states including four-wave mixing, period-one, and period-two oscillations are also observed and discussed for the microdisk laser subject to the optical injection with different wavelength detuning and optical power.

The dynamic characteristics are investigated for an 8-&mu;m-radius directional-emission microdisk laser subject to optical injection. Single mode operation with lasing mode wavelength of 1540.1 nm and the side mode suppression ratio of 35 dB is realized for the microdisk laser at the biasing current of 10 mA and the temperature of 288 K. Under the optical injection from a tunable laser, optical injection locking and the enhancement of the 3dB bandwidth of small signal modulation response from 3.4 to 13.7 GHz are observed for the microdisk laser biased at 7 mA with an injected optical power of 0.5 mW. By varying the wavelength detuning between the injection light and the lasing mode, we demonstrate the nonlinear states of four-wave mixing, period-one and period-two oscillations from the lasing spectra. Multiple peaks are observed for the period-one and period-two oscillation states.

High-speed modulation characteristics are investigated for microdisk lasers theoretically and experimentally. In rate equation analysis, the microdisk resonator is radially divided into two regions under uniform carrier density approximation in each region. The injection current profile, carrier spatial hole burning, and diffusion are accounted for in the evaluation of small-signal modulation curves and the simulation of large-signal responses. The numerical results indicate that a wide mode field pattern in radial direction has merit for high-speed modulation, which is expected for coupled modes in the microdisk lasers connected with an output waveguide. For a 15-μm-radius microdisk laser connected with a 2-μm-wide output waveguide, the measured small-signal response curves with a low-frequency roll-off are well in agreement with the simulated result at a 2-μm radial width for the mode intensity distribution. The resonant frequencies of 7.2, 5.9, and 3.9 GHz are obtained at the temperatures of 287, 298, and 312 K from the small-signal response curves, and clear eye diagrams at 12.5 Gb/s with an extinction ratio of 6.1 dB are observed for the microdisk laser at the biasing current of 38 mA and 287 K.

High speed modulation characteristics are investigated for microcircular lasers connected with an output waveguide theoretically and experimentally. The injection current profile and carrier spatial hole-burning and diffusion are accounted in the rate equation model by radially dividing the microcircular resonator into two regions under the approximation of uniform carrier densities. The numerical results indicate that wide mode field pattern in radial direction has merit for high speed modulation, which is expected for coupled modes in circular microlasers connected with an output waveguide. Small signal response curves and large signal modulation responses are investigated for a 15-&mu;mradius microlaser connected with a 2 &mu;m wide output waveguide. The highest resonance frequencies of 7.2, 5.9 and 3.9 GHz are obtained at the temperatures of 287, 298 and 312 K from the small signal response curves, and clear eye diagrams at 12.5Gbit/s with an extinction ratio of 6.1 dB are observed for the microlaser at biasing current of 38 mA and the temperature of 287 K.

Three-dimensional circular resonators connected with an output waveguide were simulated by the three-dimensional finite-difference time-domain (FDTD) technique. For the microcircular resonator with vertical waveguiding consisted of active layer confined by upper and lower cladding layers with the refractive indices of 3.4 and 3.17, the mode Q factors are greatly influenced by the thickness of the upper cladding layer. The numerical results of the near field and the farfield patterns indicate that the vertical waveguide with semiconductor materials does not provide enough optical confinement for the confined modes in the resonator. Furthermore, the lasing spectra and far-field patterns are measured for a circular microlaser with a radius of 15 &mu;m and a 2-&mu;m-width output waveguide. Single mode operation with the side mode suppression ratio up to 33 dB is realized at room temperature, and multiple peaks are observed in the vertical far-field pattern due to the vertical radiation of the mode field.

Unidirectional-emission microlasers are greatly demanded for photonic integrated circuits and optical interconnection. In
this paper, the mode characteristics of circular and coupled-circular microresonators with a bus waveguide are
numerically simulated by finite-difference time-domain technique. For a circular microresonator connected with a bus
waveguide, coupled-mode between two whispering-gallery modes can have high mode Q factor for realizing
unidirectional-emission lasing. In addition, symmetry and antisymmetry coupled modes are analyzed for the coupledcircular
microresonator with a middle bus waveguide. Furthermore, the output characteristics of a coupled-circular
microlaser, which is fabricated by standard photolithography and inductively-coupled-plasma (ICP) etching techniques,
are reported. Single mode operation is realized for the coupled-circular microlaser with a radius of 20 &mu;m and a 2-&mu;mwidth
bus waveguide.

Wavelength-scale defected circular microresonators with laterally confined metal layer are designed for directional
emission from high <i>Q</i> confined modes by boundary element method (BEM), which is firstly applied to the multilayer
structures. The influence of metal layer thickness on the mode filed patterns and <i>Q</i> factors are simulated. The results
indicate that the thickness of the metal layer has a great effect on far-field emission patterns and the mode <i>Q</i> factors.

Multiple-port directional emission microlasers are potential light sources and optical signal processing units in photonic
integrated circuits. Connecting bus waveguides to a microresonator is a simple method to realize directional emission
microlasers. In this paper, we investigate square and circular resonator microlasers connected with multiple bus
waveguides. The mode characteristics of the microresonators connected with multiple bus waveguides are simulated by
finite-difference time-domain technique, and the numerical results of mode Q factors and output coupling efficiencies
show that high efficiency directional emission microresonator lasers can be realized. Furthermore, the microcylinder
laser connected with a bus waveguide fabricated by planar technology processes is reported, and the lasing spectra of
square microlasers with four vertices connected to bus waveguides are analyzed.

Square microresonators with circular corners and a center air hole are simulated by the finite-difference time-domain
(FDTD) technology and Padé approximation method. The results show that the deformed square resonators can increase
mode Q factor at certain conditions. Furthermore, the hole in the square resonator can not only enhance the Q factor and
the output coupling efficiency, but also have the ability to select the lasing mode.

Microcavity lasers with whispering-gallery modes (WGMs) are potential light sources for photonic integrated circuits.
However, the direction emission and output power are greatly limited for microdisks with the WGMs confined by total
internal reflection. Deformed microdisk with chaos mode light rays or evanescently coupled waveguide were used to
realize directional emission microlasers. Different from the microdisk with traveling wave WGMs, confined modes in
triangle and square microcavities are standing wave WGMs. So the confined modes can still have high Q-factors if an
output waveguide is directly connected to triangle and square microcavities at the position with weak mode field
distribution. Based on theoretical analysis and numerical simulation of the mode characteristics, we fabricate directional
emission triangle and square InGaAsP/InP microcavity lasers with an output waveguide directly connected to the
resonators by standard photolithography and inductively coupled plasma etching technique. Continuous-wave (CW)
electrically injected InGaAsP triangle and square microlasers are realized at room temperature for the triangle
microlasers with side length from 10 to 30 &mu;m and the square microlaser with the side length of 20 &mu;m.

Directional emission semiconductor microcavity lasers integrated with semiconductor planar technique are potential light
sources for photonic integrated circuits. In this article, we investigate mode characteristics for equilateral triangle and
square microcavities for realizing directional emission microcavity lasers. The analytical mode field distributions and
mode wavelengths are presented for two-dimensional (2D) triangle and square resonators, and directional emission are
simulated by finite-difference time-domain (FDTD) technique. The numerical results of mode Q-factors and output
coupling efficiencies for the triangle and square resonators show that high efficiency directional emission microcavity
lasers can be realized by directly connected an output waveguide to the resonators, because the mode field patterns are
standing distributions in the triangle and square resonator. Laser output spectra of fabricated InGaAsP/InP triangle lasers
are compared with the analytical results.

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